The present invention is generally directed to radiocommunication systems and, more particularly, to techniques and apparatus for muting a microphone used in radiocommunication systems.
The cellular telephone industry has made phenomenal strides in commercial operations both in the United States and the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as to maintain high quality service and avoid rising prices.
In addition to the challenges posed by a need for greater system capacity, the designers of the remote units used in radio communication systems, e.g., mobile phones, have their own unique set of challenges. For example, a well known challenge in the industry is to extend the time between recharging batteries used to power these remote units, while at the same time continuing to provide additional functionality to satisfy subscriber demand. These competing desires lead remote unit designers to search for creative ways in which to further reduce energy consumption in remote units.
As the radiocommunication industry matures, various subscriber usage patterns have been recognized, For example, it has been found that during a typical voice connection between two subscribers, the actual voice activity transmitted over the air interface accounts for only about 35% of the total connection time. Accordingly, remote units have been designed to take advantage of this fact by actuating the transmission circuitry only during the voice-active portion of a call and rendering the transmission circuitry inoperative during periods of silence. This has been implemented, for example, using a detector for detecting voice activity and a discontinuous transmitter which becomes inoperative when the voice activity detector detects a pause in the user's speech,
While this technique has been shown to reduce the power consumption of remote units by turning off the transmitting circuitry for extended periods of time, it poses a problem for reproduction of the voice signal at the receiving side. Specifically, since the transmitter is turned off during periods of silence, the background noise which would otherwise be transmitted over the air interface is not received by the other subscriber's equipment. This, in turn, results in a reproduced voice which is discontinuous and includes audible artifacts, e.g., popping sounds.
One way to overcome this difficulty is to generate artificial background noise for reproduction at the receiving side when no voice signal is transmitted. This artificial background noise is commonly referred to in the art as "comfort noise". Comfort noise can be generated by adaptive functions which monitoring the background noise picked by the microphone of a remote unit. When a pause in speech is detected, the comfort noise functions generate comfort noise information which is transmitted over the air interface instead of speech codes. This information takes relatively little time to transmit, thereby allowing the transmitter to be turned off during most of each period of silence, At the receiving end (or at the base station), the comfort noise information is used to generate background noise so that the listener is not troubled by the discontinuity in transmission.
The comfort noise functions resident in the transmission circuitry of the remote stations are, as described above, adaptive in nature so that they gradually adjust the comfort noise information to reflect changes in the background noise. Thus, the comfort noise functions need to continuously receive information from the microphone in order to accurately determine the background noise to be replicated at the receiving side.
Another function which subscribers desire in remote terminals is the capability to selectively mute transmissions, e.g., by pressing a mute button. Conventionally, this function has been provided by interrupting the signals transmitted from: the microphone to the rest of the transmitting circuitry. This muting solution is problematic, however, from the point of view of the comfort noise functions, since these functions then receive only complete silence from the microphone. This leads to background noise being generated at the receiver which has a lower level than expected by the listener.
Thus, it would be desirable to provide for a muting function which allows a user to selectively mute transmissions from a remote unit, but which does not affect the ability of the system to accurately gauge and reproduce background noise during periods of silence in a conversation.